tf should copy its arguments; instead they are mutated

``` python
import numpy as np
import control

num = [[[1]]]
den = [[[1,1]]]

print('num before',num)
g = control.tf(num,den)
print('num after',num)
```

outputs

```
num before [[[1]]]
num after [[array([1])]]
```

expect num to be unchanged.
